I. Field of the Invention
The present invention relates generally to a sealing device for use in industrial mixing or blending equipment of a class which includes one or more rotary mixing blades having long shafts extending into a large mixing bowl in which viscous liquids, particularly explosive propellant materials, are mixed. More particularly, the present invention provides such a mixer with a retractable mixing blade shaft sealing gland system that can be withdrawn and cleaned and reset between mix cycles and which, in addition, minimizes the clearance between the gland and the mixer bowl thereby reducing the material lost during the mix cycle and improving batch-to-batch solvent content consistency, particularly in high-solvent mixes
II. Related Art
Mixers and blenders for homogenizing hazardous materials such as propellant materials presently use outboard bearings spaced from the mixing bowl as supports for rotating shafts of mixing blades to segregate areas where mechanical friction is present from areas where the propellant is present inside the mixer bowl during normal operation. Sealing the interface between the stirring shafts and the base openings where the rotating shafts emerge from the bowl has always been difficult. The mixers are generally modified versions of equipment that has been used in the baking industry where sealing has also been a problem. This has led to the development of sealing systems of various kinds. One such system as applied to a dough kneading device is illustrated and described in U.S. Pat. No. 4,412,747. Another type of shaft sealing device is shown in U.S. Pat. No. 4,858,936.
Owing to the hazardous mixture of the material, propellant mixers have had to be adapted to function safely in an environment involving different unique concerns. The standard practice for sealing stirring shafts in the propellant/explosives industry had been to tightly seal the gland area using a flax/felt packing material. However, these gland areas tended to become contaminated by propellant fines and subsequent solvent loss and viscous heating caused drying of the propellant fines and this, in turn, increased the danger of ignition and subsequent propellant fires in the mixing operation.
More recently, the practice has been to fully open the gland area by removing the packing material and allowing migrating product to flow freely through a fixed gap in the gland area into a catch pan beneath the stirring shaft attempting to keep losses to a minimum. A gland assembly drawing illustrating this prior art concept appears in FIG. 1.
FIG. 1 is a fragmentary view of a mixing system showing a prior art gland assembly partially in section. It includes a schematic representation of a mixing bowl-fragment 10. Spaced parallel, oppositely rotated mixing blades 12 and 14 are fixed to and driven respectively by stirrer shafts 15 and 16, which are supported by a bearing plate 18 spaced at a distance from the mixing bowl end plate 20. Stationary sealing glands 22 and 24, respectively, surround the shafts 15 and 16 and are bolted to the mixer end plate 20 as at 26 and 28, respectively. Gaps indicated by 30 are provided and maintained surrounding the mixer shafts between the shafts and the glands. The gaps are left fully open allowing migrating product propelled by the mixing action toward end wall 20 to seep or flow freely through the gaps in the gland area. Product flow force is indicated by arrows 32 and mixing blade rotation is indicated by arrows 34 and 36. A catch pan (not shown) is provided beneath the stirring shafts to catch product that migrates past the glands.
While this has solved some prior problems, it has been found, however, that with mixes of high solvent content, excessive amounts of product can be lost oozing through the glands or, in the worst case, the glands may become packed with material. The resulting friction, of course, can be hazardous. This situation necessitates that the glands (which are assembled from two semicircular halves) be disassembled and cleaned after each mix cycle to minimize the risk of buildup.
In view of the present situation, there exists a definite need for innovative technology with regard to improving shaft sealing gland systems in such mixing devices, both from the standpoint of safety and minimizing down time while preserving the integrity of each mix.